A new loss-of-mains detection method for use with very small generators is proposed. A known load is repeatedly switched on to the supply, and the resulting voltage changes used to estimate the source impedance, and hence to detect loss of mains. Correlating the load switching with the voltage change prevents false operation arising from random load changes. Experiments and simulations are discussed which demonstrate the principles of the technique. Test results are presented which show that the technique operates correctly, and can reliably detect a loss of mains without being susceptible to false tripping.